Selective galvanizing of steel strip



g- 1970 R. G. BAIRD-KERR E L 3,523,067

SELECTIVE GALVANIZING OF STEEL STRIP Filed May 27. 1968 OOOOOOOOO INVENTORS RICHARD G. BAlRD-KERR ROBERT F. HUNTER United States Patent SELECTIVE GALVANIZING OF STEEL STRIP Richard G. Baird-Kerr and Robert F. Hunter, Burlington,

Ontario, Canada, assignors to The Steel Company of Canada, Limited Filed May 27, 1968, Ser. No. 732,160 Int. Cl. C23b 5/48; C23f 17/00; B44d 1/52 US. Cl. 204-45 Claims ABSTRACT OF THE DISCLOSURE Galvanizing of only selected areas of ferrous strip or sheet is achieved by providing the surface area where no galvanizing is desired with a coating which will not be wetted by molten zinc. The coating is a composite of chromium metal and hydrated chromium oxide and is formed by electrodeposition. After passage through the galvanizing bath there is no need to remove the barrier coating. The coating provides a clean surface which is amenable to painting.

BACKGROUND OF THE INVENTION The present invention relates to a method of galvanizing a selected surface of a ferrous strip or sheet. More particularly, the present invention includes the step of forming a composite coating on a selected surface of a ferrous strip or sheet (the surface which is not to be galvanized) prior to hot dip galvanizing in a manner which prevents both the oxidation of the base plate of, and the adherence of metallic zinc onto the selected surface, and results in a surface which has both high corrosion resistance and excellent paint adhesion.

The automotive industry has been concerned with the corrosion of steel in cars. With the increasing use of salt on highways during the winter months, the rate of corrosion of rocker panels and fenders of cars has been greatly accelerated. Galvanized steel was considered the best material to use in car construction for corrosion protection. The introduction of galvanized steel, however, raised two ditficulties:

(i) Poor paint match between adjacent painted galvanized and painted cold rolled steel surfaces due to surface texture differences, and

(ii) Poor weldability (spot-welding) of galvanized steel surfaces.

Automatic designers realized that one way to overcome these problems would be to use a steel sheet that was galvanized on one side only, i.e. the side facing the interior of the car. The exterior steel sheet surface would be protected by phosphatizing and painting in the conventional manner.

Various means for producing a selectively galvanized product, or one-side galvanized sheet, have subsequently been devised. These means can be classified as follows:

(i) Processes wherein zinc is selectively applied to one side of the steel sheet using electrolytic, electrophoretic or vacuum vapour deposition techniques. These methods might fully satisfy the requirements of the automatic industry, but the cost of applying sufficient zinc using these techniques is too high to encourage extensive use of a product made in this way.

(ii) Processes wherein the molten zinc comes in contact with only one side of the strip by utilizing special mechanical coating apparatuses, e.g. US. Pat. No. 3,228,788, A. Teplitz, J an. 11, 1966. These processes have not met with commercial acceptance possibly due to the special equipment required and diflicult operating problems envisaged in maintaining the coating equipment in continuous service.

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(iii) Processes wherein both sides are coated with zinc using hot-dip galvanizing methods, and subsequently removing the zinc from one side by acid dissolution, e.g. US. Pat. No. 3,178,305, A. H. Ward, Apr. 13, 1965. These processes would be costly and difiicult to implement on a commercial basis. They too, have failed to meet with commercial acceptance.

(iv) Processes wherein the molten zinc comes into contact with both sides but is prevented from coating one side due to the presence of a non-wetting barrier film on the surface, e.g. U.S. Pats. Nos. 2,894,850, I. L. Greene et al., July 14, 1959, and 3,149,987, R. L. C'randall, Sept. 22, 1964. Although one-side galvanized material has been produced commercially using such processes, they have to date met with only limited success. Commercial success using these means has been limited because:

(a) the barrier film must be removed after galvanizing since it interfers with subsequent phosphatizing and painting operations.

(b) the barrier film after galvanizing, is often diflicult to remove without either marring (e.g. scratching) the surface of the steel or leaving a film of residue on it, neither of which are acceptable to the automotive industry for aesthetic and practical reason respectively.

It is the basic object of this invention to provide a process for hot-dip galvanizing steel strip which will give a product which is fully zinc coated on one surface for corrosion protection, and uncoated with zinc on the opposite surface and in a clean state for subsequent processing.

It is a further object of the present invention to provide a process that obviates the necessity of removing the barrier film after galvanizing, yet still leaves a clean surface amenable to painting.

BRIEF SUMMARY OF THE INVENTION The present invention may be generally defined as an improvement in the method of galvanizing only selected areas of a ferrous material strip or sheet which comprises electrodepositing on surface areas which are not to be galvanized, a composite coating of chromium metal and hydrated chromium oxide, and subsequently passing the thus treated strip or sheet through molten zinc. The bath conditions may be such that both the chromium metal and sufficient chromium oxide are deposited in a single step, or a metal forming bath can be followed by an oxide-forming bath. The coating process can be considered as a pre-treatment to hot dip galvanizing and is normally followed by a flux pre-treatment.

Prior to the electrolytic treatment, the surface of the steel should be cleaned by any conventional treatment such as degreasing with alkalis and pickling with acids. After the deposition of the coating, the coated surface should be rinsed and dried prior to dipping in the molten zinc. Flux is preferably applied hot by spraying or by contact applicators to the untreated side of the sheet just prior to the drying step. However, the whole strip may be immersed in a galvanizing flux, and the flux thereafter removed from the coated side. The sheet is then pre-heated, zinc dipped and allowed to cool. The basic steps of cleaning, rinsing, pickling, rinsing, plating, rinsing, fluxing, drying, pre-heating, zinc dipping and cooling, can be compared to those presently employed on a pre-fiux type galvanizing line, e.g. Canadian Pat. No. 501,746, Cook et al., Apr. 20, 1954, such as cleaning, rinsing, pickling, rinsing, fiuxing, drying, pre-heating, zinc dipping, cooling and chemical post-treatment. In the present invention, it may also be of advantage to dry the coated steel strip after the plating rinse in order to minimize abrasion of the chrome oxide on the line rolls. The immediate drying 3 of" the coating tends to strengthen or cure the gel-like OXldBr' Y a BRIEF DESCRIPTION OF DRAWING An apparatus for carrying out the method of the invention is shown schematically in the attached drawing.

DETAILED DESCRIPTION Referring to this drawing in more detail, steel strip is paid off from a mandrel 1 and drawn through a first tank 2 of a .series of processing tanks. In this first tank, the strip is cleaned in a hot alkaline solution 3 and then subjected on both sides to spray rinses 4 and 5. The strip is then drawn over suitable rollers and through a pickling tank 6 containing an acid pickling solution 7. The thuscleaned strip then travels past rinsing stations 8 and 9 to a third tank 10 containing the chromium plating electrolyte 11 and tin-lead anodes 12. The tank 10 is provided with an electrically isolated submersion roll 13 which, in co-operation with conductor rolls 14 and 15 disposed above the tanks, causes the stop to follow a U-formation with the arms of the U bracketed between the anodes 12.

The recommended conditions for carrying out the electro-deposition will be discussed in greater detail later. For the moment, it sufiices to say that the lower side of the strip is coated with a barrier coating containing chromium metal and hydrated chromium oxide, hereinafter referred to as chomium-coated." The thus-coated strip is caused to pass another pair of opposed rinsing stations 1-6 and 17 and is then directed by rolls 18a and 1812 through a bath 19 of a conventional galvanizing flux. The strip is drawn vertically upward out of the fiuxing bath and through a pre-heater 20. On its way to the preheater 20 the chromium coated side of the strip is given a water rinse 21, and any excess water is removed by a conventional wiper 22. After passage through the preheater 20, the strip is drawn through a conventional hot dip galvanizing bath 23 and thence, via a series of rollers, through a cooling chamber 24. Enroute to the cooling chamber 24, the strip passes between a pair of coating rolls 25 which control the thickness of the zinc coating on the galvanized surface. The strip is finally coiled on take-up mandrel 26.

The table Which appears at the end of this specification lists different coating systems we investigated (a number of which are not in accordance with the present invention), and gives the results obtained through the use of these systems, expressed in terms of the ability of the treated strips'or sheets to resist zinc adhesion and protect the 'steel base against oxidation. The results indicated that the susceptibility to zinc pick-up is a function of the amount of chromium oxide in the coating; the higher the oxide level, the less the possibility of Wetting the surface with Zinc. The susceptibility of the base steel to oxidation appears to be primarily a function of the chromium metal level in the chromium coating; the thicker the chromium metal, the less the possibility of base plate oxidation.

The minimum amount of chromium oxide required to prevent zinc metal adhesion on the substrates normally employed for galvanizing was found to be about 2 mgm./ sq. ft. expressed as chromium in the oxide. The minimum amount of chromium metal required to prevent oxidation of the base steel was found to be about 3 mgm./ sq. ft. The maximum levels cannot be defined with respect to the objects of this invention, since any values above the minimum requirements should result in a one-sided galvanized material. However, thick chromium oxide coatings tend to become coloured and such colour can result in staining of the final product. A preferred range for the oxide can, then, be given as 2 to 10 mgm./sq. ft. expressed as chromium in the oxide. Thick chromium metal coatings crack when severely bent or drawn and this characteristic could cause failures on post-paint systems in formed areas. A preferred range for the chromium metal can, then, be given as 3 to 20 mgm./ sq. ft.

The reason for the lack of zinc adhesion to a hydrated chrome oxide film is believed to be due to the formation of a water vapour barrier film in thBJi-HQDEIIh PIOdIlCCd by dehydration of the coating. The level of hydration of the coating upon entering the zinc bath could therefore govern the efficiency of the barrier eifect. The extent of preheat just prior to zinc dipping has in fact been found to be of importance in the degree of zinc pick-up obtained. Pre-heat temperatures greater than 600 F., coupled with times of greater than 30 seconds, cause zinc pick-up on the treated surface. The zinc coverage becomes appreciable when the zinc immersion times increase to values beyond one minute. The'normal conditions in a pre-flux type galvanizing line of:'450, F. at. a residence time of 15 seconds otter no problems lD ifiichlties could be expected in the case of reducing-atmosphere typ gaivanizing lines which require preliminary proce s sing is'teps at temperatures appreciably above 600 F.-

The flux can be applied to only one sideofathe strip, or to both sides if quickly rinsed from the chromium coated surface. It has been found thatnine ammonium chloride type fluxes can be in contact with tli'echromium coating for periods of up to 30 seconds without destroying the effectiveness of the film, A.met l 1Qd of ensuring a minimum of creepage of the flux around th'e edge of the sheet is to spray the chromium coated-side-with'water while at the same time the untreated side is being sprayed withflux. v i

A preferred bath for the application of' the chromium coating in the present invention has a composition of 50 to 60 gm./litre chromic acid, 0.20 to 0.40 gm./litre sulphuric acid and 0.5 to 1.0 gm./litre fiuoboric. acid. The use of the fiuoboric acid is optional, but is generally desirable for the attainment of high plating efficiencies. The steel sheet is made cathodic at current densities of 200-300 amperes/ sq. ft. Suitable coatings are achieved in about 2 seconds at temperatures ranging from to F.

The final chromium metal-chromium oxide coating has considerably more corrosion resistance than untreated mild steel. An accelerated acid-type spot test, which has been found to correlate reasonably well with the general corrosion rating, indicates a corrosion resistance about 50 times better than untreated mild steel. Paintability tests on this surface, involving cross-hatched tape adhesion tests, falling ball impact, chip resistance and conical mandrel flexibility tests indicate excellent adhesion; equivalent to that obtained on phosphated surfaces. This method then, in addition to providing ameans of galvanizing one side of a sheet, has generated "asurfacle which is an excellent base for paint. The removal of the barrier coating, used for making one-sided galvanized products, is thus not required for paint application. H The chromiummetalachromium oxide coating applied .in the method of the invention to prevent zinc pick-up on one side of the ferrous strip or sheet which is to be subjected to hot dip galvanizing may beapplied using the electrolytes and electrodeposition procedures disclosed in Canadian Pat. No. 710,309, Kitaniur'a et aL, "May 25, 1965. We do not, however, find it necessary for our present purposes to use in the electrolyte any of the water soluble reducing agents recommended in the said patent.

EXAMPLES A number of examples will now be set out in order to further illustrate the method of the invention.

Example 1 and 50 ml./litre of a commercial alkali de-ruster. After washing with water, the strip was then transported to an electrolytic bath and direct current was applied. The steel strip was made cathodic relative to lead-tin (Pb 93:Sn 7) anodes. The anode was so disposed that only one side of the sheet received electrolytic treatment. This was accomplished by causing the strip to take a U-formation in the electrodeposition bath, with a tin-lead anode being located in the bath opposing an outer side of each arm of the U, as shown in the drawing. The composition of the bath was 55 gm./litre of chromic acid, 0.30 gm./litre sulphuric acid and 0.50 gm./litre fiuoboric acid (48 percent). The temperature was 118 F., with a current density of 280 amperes/sq. ft. and a treatment time of two seconds. The strip was next rinsed with warm water (138 F.), dried, then re-cleaned for six seconds at 190 F. in a solution containing 45 gm./litre of a commercial alkali cleaner. After rinsing with water, a 10 B. solution of zinc ammonium chloride flux at 200 F. was sprayed onto the untreated side and this lightly brushed to remove excess flux. During the flux spraying, the chromium coated side of the strip was spray rinsed with water at 130 F., and the excess water was removed by a rubber blade wiper. The strip was next dried and preheated for seconds in an oven at 450 F., after which it was immediately immersed in molten zinc for about four seconds at 870 F. The zinc bath contained 0.12 percent aluminum and 0.36 percent lead. The strip was then allowed to cool.

The final product from this overall treatment consisted of a material having an average of 0.45 oz./sq. ft. of zinc metal on one side and a composite chromium metalchromium oxide coating on the opposite side. The composite chromium coating successfully prevented the sticking of the metallic zinc and the bluing oxidation of the steel substrate upon emergence from the molten zinc. The appearance of the chromium coating in fact changed little during the entire operation, remaining essentially clear and metallic in lustre. The only change noted was the development of a very light brown mottle pattern after zinc dipping. The material causing the mottle has been analysed as zinc oxide in very low concentration. Some metallic zinc pick-up occurred on the chromium coated side within 0.25 inch of each edge of the sheet, owing to the creepage of the flux from the opposite side. Another type of edge effect which was noted resulted from creepage of the rinse water applied to the chromium coated side during fiuxing, giving a bare edge on the galvanize side.

The zinc coating was tested for adhesion by performing envelope bends and examining the degree of cracking or coating loss. The results indicated that a conven tional galvanize coating was achieved on the one side.

The chromium coating was analysed before and after immersion in the zinc. The chromium oxide level, reported as chromium in the oxide, amounted to 5.5 mgm./ sq. ft. after plating and 0.84 mgm./sq. ft. after zinc dipping. The metallic portion of the coating increased from 6.9 to 7.6 mgm./ sq. ft. of chromium.

The corrosion resistance of the chromium coated side was tested by use of an acid-type spot test. The results indicated an iron dissolution value of 108 gm. Fe/ cm. after complete processing. This value is about 50 times lower than that obtained on uncoated mild steel (i.e. 5016 gm. Fe/cmP). Prior to immersion in the molten zinc the coating gave an iron dissolution value of 83.8 gm. Fe/20 cm.

Paintability tests on the chromium coated side indicated that conventional primers and automobile grade paints gave builds and adhesion properties equivalent to that of phosphated metal surfaces. Paint tests which were performed included cross-hatched tape adhesion, falling ball impact, chip resistance and flexibility using a conical mandrel.

Conventional commercial phosphating solutions did not react with the chromium coated surface. Spectrographic analysis indicated that little or no phosphate was deposited, with the only effect being a cleaning action on the sheet and removal of the aforementioned light brown mottle. Little or no chrome was lost to the phosphate solution. Paintability tests on the material indicated adhesion properties equivalent to those obtained on the product not subjected to the phosphating process.

Example 2 The same kind of steel sheet was cathodically cleaned as described in Example 1, then water rinsed, derusted for three seconds at room temperature in nine percent sulphuric acid and again rinsed with water. The sheet was then placed in an electrolyte bath and subjected to the same conditions as described in Example 1, with the one exception of a lower current density (230 amperes/sq. ft.). After warm water rinsing, the sheet was wiped on the uncoated side with 10 B. zinc ammonium chloride flux at 200 F. The sheet was then dried and pre-heated, dipped in molten zinc and cooled in the same manner as described in Example 1.

Once again a sheet was obtained which was galvanized on one side and had a composite chromium metal-chromium oxide coating on the other side. The zinc thickness was 0.6 oz./sq. ft. and the chromium coating contained 1.49 mgm. Cr/sq. ft. in the oxide and 6.22 mgm. Cr/ sq. ft. present as the metal. Prior to hot zinc dipping, the oxide and metal levels were 4.75 and 3.93 mgm. Cr/sq. ft. respectively. The chromium coated side had a corrosion resistance and paintability similar to that described in Example 1.

Example 3 The same kind of steel was subjected to the same sequential operations as described in Example 2 except that the flux was applied to both sides of the sheet by a dipping technique, then removed from the chromium coated side by spray rinsing with water. The same results were achieved as in Example 2, indicating that for limited contact times 20 seconds), the zinc ammonium chloride flux does not substantially affect the chromium coating.

Example 4 The same kind of steel was subjected to the sequential operations as described in Example 2 except that the plating conditions were changed to a bath composition of 250 gm./litre of chromic acid and 2.5 gm./litre sulphuric acid. The temperature was F., with a current density of 233 amperes/sq. ft. and a treatment time of three seconds. These plating conditions gave oxide and metal levels of 2.3 and 13.3 mgm. Cr/sq. ft. respectively. The coating was non-wetting to zinc and resisted discolouration in a similar manner to the coating described in Example 2.

Example 5 The same kind of steel was subjected to the sequence of steps described in Example 2. The pre-heat temperature just prior to zinc dipping was, however, varied from 450 to 650 F., with time held constant at 30 seconds. The zinc began to stick to the coated side of the strip when the pre-heat temperature exceeded 600 F. The amount of zinc adhering to the chromium coating at 600 F. increased with the time of pre-heating. For example, very little zinc metal adhered when the pre-heat time was 35 seconds. However, when the sheet was held at 600 F. for 1 to 5 minutes, zinc metal pick-up amounted to 25 to 50 percent of the area of the sheet.

Example 6 The same kind of steel sheet was cleaned for 6 seconds at F. in a solution containing 45 gm./litre of a commercial alkali cleaner, then water rinsed and de-rusted for three seconds at room temperature in a 10 percent by weight hydrochloric acid solution. After rinsing with water, the sheet was placed in an electrolyte bath as described in Example 4 for two seconds at the same temdiscolouration in a similar manner to the coating described 10 in Example 2.

strip is immersed in a galvanizing flux prior to its passage through the molten zinc, and the flux material is rinsed and/or wiped from the chromium coated surface before the strip is passed through the molten zinc.

6. The improvement defined in claim 2, in which the electrodepositing step is carried out in an electrolytic bath containing from about 50 to about 60 grams per litre of chromic acid and from about 0.20 to about 0.40 gram per litre of sulphuric acid.

7. The improvement defined in claim 2, in which the electrodepositing step is carried in an electrolytic bath TABLE OF CHROMIUM CONTAINING SYSTEMS TESTED FOR SELECTIVE GALVANIZING Coating composition Cr Method Cr oxide, Molten zinc of mgm.) mgmJ Wettability for 5 present System Treating solution Method of application sq. ft. sq. ft. second immersion Resistance to oxidation invention Phosphate chromate Phosphoric Acid: 1% Dip 5 seconds 110 F..- 0. O Wetting Poor No.

coating. by weight. Potassium dichromate: 1% by Weight. Chromate cating. Two solutions: 1) Sodi- First solution: electro- 0. 0 N on-wetting ..do No.

um dichromate: 22.5 lytic, 1 second cagmJl. (2) Chromic thodic, 50 a./sq. 12., acid: 8.0 gm./l. 170 F. Second solution: Dip 1 second, 180 F. Conventional chro- Chromic acid: 250 gm./l. Electrolytic, 4 seconds 21. 0 l. 7 Slight wet-ting Excellent... N0.

mium plating. Sulphuric acid: 2.5 cathodic, 233 a.lsq.

grn./l. ft., 185 F. Do do Electrolytic, 25cc 9. 0 1.7 do .do No.

onds cathodic, 233 also. ft., 188 F. Conventional chromi- Chromic acid: 250 gmJL, Electrolytic 2 seconds 9. 0 1. (H. Some wetting .dc No.

um plating followed Sulphuric acid: 2.5 cathodic, 233 a./sq. by partial stripping gmJl. stripping soluit... 185 F. Followed of the oxide. tion 10% hydrochloric by 2 seconds immeracid. sion in cold 10% hydrochloric acid. Do Chromic acid: 250 gm./l., Electrolytic 4 seconds 15. 8 1.6 Slight wetting .do. No.

Sulphuric acid: 2.5 cathodic, 233 a./sq. gm./l., Stripping soluft., 185 F. Followed tion hydrochloric by 1 second immeracid. sion in cold h ydrochloric acid. Conventional chromi- Chromic acid: 250gm., l., Electrolytic 3 seconds 13. 3 2. 3 N oil-wetting .do Yes um plating. Sulphuric acid: 2.5 cathodic, 233 a.,"sq.

gmJl. ft., 185 13. Do Chrornic acid: 350 gmJL, Electrolytic 4 seconds 15. 09 9. 45 .do Some chromate stain- Yes.

Sulphuric acid: 1.2 cathodic 266 also.

NaiSiFs 9.35 ft., 54 F.

gm. Modified chromium Chromic acid: 55 gm.[l., Electrolytic 2 seconds 4. 1 4. 1 do Excellent Yes.

plating (oxide foriu- Sulphuric acid: 0.24 cathodic 233 a., 'sq. ing electrolyte). gm./l., Fluoboric acid: it, 120 F.

0. 5 gm./l. Do do, .do 3.9 4.8 do do Yes. Do do Electrolytic lsccoud 2. 6 2. 8 do ir r'cs.

cathodic, 233 aJsq. it, 120 F. Do "do .do 2.8 3.1 .do Good excellent Yes Do Chromic acid: 55 gm./l., Electrolytic 1.2 sec- 4. 6 3. 8 do Exccllent Yes) Sulphuric acid: 0.3 onds cathodic 335 gnu/1., Fluohoric acid: aJsq. it, 120 F.

0.5 gut/1.

1 Oxidation at pinholes. 2 Preferred.

about 2.0 mgm. per square foot of chromium oxide, ex-

pressed as chromium in the oxide.

3. 'The improvement defined in claim 1, in which the coating is preheated to a temperature not exceeding about 600 F. prior to immersion in molten zinc.

fiQThe improvement defined in claim 3, in which the coating is'maintained at said preheat temperature for a period of time not exceeding about 30 seconds.

'5. The improvementdefined in claim 1, in which the containing from about 50 to about 60 grams per litre of chromic acid and from about 0.5 to about 1.0 gram per litre of fluoboric acid.

8. The improvement defined in claim 2, in which the electrodepositing step is carried in an electrolyte bath containing from about 50 to about 60 grams per litre of chromic acid and from about 0.20 to about 0.40 gram per litre of sulphuric acid, and in which the ferrous sheet or strip is made cathodic at current densities of from about 2 00 to about 300 ampercs per square foot.

9. The improvement defined in claim 2, in which the electrodepositing step is carried in an electrolyte bath containing from about 50 to about 60 grams per litre of chromic acid and from about 0.20 to about 0.40 gram per litre of sulphuric acid, and in which the ferrous sheet or strip is made cathodic at current densities of from about 2 00 to about 300 amperes per square foot for a period of a few seconds at a bath temperature between and about 10. A method for one side galvanizing of a sheet or strip of ferrous material which comprises cleaning said material, electrodepositing on one side thereof a dense adherent coating containing chromium metal and hydrated chromium oxide by making said material cathodic to an inert anode in an electrolytic bath containing about 55 gm./ litre of chromic acid, about 0.30 gm./1itre sulphuric acid and about 0.50 gm./ litre of fiuoboric acid, using a current density of about 280 amperes/sq. ft. for a period of about 2 seconds at a bath temperature of about 118 F washing the thus-coated strip, applying a conventional galvanizing flux to the uncoated side of said material, drying and preheating said strip, and immersing it for a few seconds in a hot dip galvanizing bath.

References Cited UNITED STATES PATENTS 3,081,238 3/1963 Gurry 204-28 3,104,993 9/1963 Sievert 117-55 5 3,177,085 4/1965 Adams 1175.5 3,178,305 4/1965 Ward 204-15 3,383,250 5/1968 Pierson et a1. 117-5.5

JOHN H. MACK, Primary Examiner T. TUFARIELLO, Assistant Examiner US. Cl. X.R. 

